Apparatus and method for fabricating holey optical fiber

ABSTRACT

There is provided an apparatus and method for fabricating a holey optical fiber. An optical fiber with air holes of a predetermined size and shape along the length of the optical fiber is drawn by supplying nitrogen gas into air holes through one end of a holey optical fiber preform while heating the other end of the preform.

CLAIM OF PRIORITY

[0001] This application makes reference to, incorporates the sameherein, and claims all benefits accruing under 35 U.S.C. Section 119from an application for APPARATUS AND METHOD FOR FABRICATING HOLEYOPTICAL FIBER, filed with the Korean Industrial Property Office on Apr.18, 2000 and there duly assigned Serial No. 20285-2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to an optical fiber, andmore particularly to an apparatus and method for fabricating an opticalfiber having a plurality of air holes.

[0004] 2. Description of the Related Art

[0005] A holey optical fiber consists of a plurality of submicron-sizedair holes running the length of a silica fiber in the cladding layer toconfine light (by modified total internal reflection) to a core layer.The holey optical fiber finds its increased applications as a waveguidewith novel properties for dispersion compensated fiber communications,non-linear fiber and grating applications, and optical fiberamplifications.

[0006] Basically, the holey optical fiber includes a dielectricstructure with a refractive index that varies periodically across atransverse plane but is uniform in the normal direction. This dielectricstructure causes Bragg diffraction and allows the holey optical fiber tohave a “photonic band gap” at a specific wavelength or with respect to alight wave propagation direction. The “photonic band gap” refers to acondition in which light of certain frequencies will not propagate inthe material and is analogous to the familiar electronic band gap,except that it applies to photons instead of electrons.

[0007] Accordingly, light can behave in unfamiliar ways, traveling alongthe holey optical fiber due to the photonic band gap effect and thereflective index characteristics. For details, see T. A. Birks, et. al.,Electronic Letters, vol. 31(22), p. 1941, October, 1995 and J. C.Knight, et. al., Proceeding of OFC, PD 3-1, February, 1996.

[0008] In the conventional method of fabricating the holey opticalfiber, an optical fiber preform is typically formed by arranging aplurality of hollow cylindrical glass tubes with a predetermined formalong the cladding layer. At the same time, a core preform rod isinserted to be used depending on the application purposes, i.e., as anoptical fiber amplifier, an optical fiber grating, or a non-linearoptical fiber. Thereafter, the ends of the glass tubes are sealed, thenthe optical fiber is drawn from the preform. Accordingly, the resultingoptical fiber has a plurality of submicron-sized air holes in thecladding.

[0009] In the conventional holey optical fiber, however, the outer airholes are typically closed or are much smaller than the inner air holesof the fiber. Hence, during the drawing of an optical fiber from thepreform, relatively large inner air holes are transformed to an ovalshape since the outer glass tubes are melted faster than the inner glasstubes due to the difference in the heat conductivity between the innerportion and the outer portion of the optical fiber preform. This type ofdistortion in the air holes makes the continuous mass production ofholey optical fibers very difficult.

SUMMARY OF THE INVENTION

[0010] It is, therefore, an object of the present invention to providean apparatus and method for fabricating a holey optical fiber byvertically arranging a plurality of glass tubes in a gel to prevent thedistortion of air holes during the drawing step of the optical fiber.

[0011] According to another aspect of the invention, the method forfabricating the holey optical fiber is executed as follows. A sol isfirst formed by mixing a starting material, deionized water, and anadditive. The sol is filled into a circular frame and gelled, and apreform rod is inserted into the center of the resulting gel. Meanwhile,a plurality of glass tubes is vertically arranged around the preform rodin the gel. Then, the gel is removed from the circular frame and dried.The dry gel is glassified through a heat application during thesintering process. Thereafter, the holey optical fiber is drawn from theholey optical fiber preform resulting from the sintering process bysupplying gas into the ends of the air holes in the holey optical fiberpreform while heating the other ends of the air holes.

[0012] According to further aspect of the invention, the apparatus forfabricating the holey optical fiber, as described in the precedingparagraphs, is installed by the following means. One end of the holeyoptical fiber preform is sealed with a preform cover. A gas suppliersupplies gas into the preform cover. A pressure regulator regulates theamount of gas supplied from the gas supplier to be constant. A heater isinstalled at the other end of the holey optical fiber preform forheating the other end of the preform to draw an optical fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The above and other objects, features, and advantages of thepresent invention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

[0014]FIG. 1 is a perspective and plan view of a holey optical fiberpreform according to a preferred embodiment of the present invention;

[0015]FIG. 2 is a flowchart illustrating a method of fabricating a holeyoptical fiber according to the preferred embodiment of the presentinvention; and,

[0016]FIG. 3 is a schematic view of a holey optical fiber fabricatingapparatus according to the preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] A preferred embodiment of the present invention will be describedhereinbelow with reference to the accompanying drawings. For the purposeof clarity, well-known functions or constructions are not described indetail as they would obscure the invention in unnecessary detail.

[0018]FIG. 1 is a perspective and top view of a holey optical fiberpreform according to a preferred embodiment of the present invention.Referring to FIG. 1, the holey optical fiber preform 10 includes acladding 12 and a core 14. The cladding 12 has an array of air holes 16.The cladding 12 exhibits a lower refractive index than the core 14 andis formed by a pure silica or a fluorine-doped silica. The core 14 showsa higher refractive index than the cladding 12. The core part 14 may beformed by an erbium or germanium-doped silica. The air holes 16 arearranged in a hexagonal shape similar to a beehive, but it should beunderstood that different shapes are possible in the arrangement of theair holes 16.

[0019]FIG. 2 is a flowchart illustrating a holey optical fiberfabricating method according to the preferred embodiment of the presentinvention. Referring to FIG. 2, the holey optical fiber fabricatingmethod includes the steps of sol formation (step 100), sol filling (step200), preform rod installation (step 300), arrangement of a plurality ofglass tubes (step 400), drying of a gel (step 500), gel sintering (step600), and gas supplying/optical fiber drawing (step 700).

[0020] In step 100, a sol is formed by mixing a starting material, adeionized water, and an additive. The starting material may be famedsilica or silicon alcoxide. The additive can be a dispersing agent, acatalyst, or a binder.

[0021] In step 200, the sol prepared in step 100 is poured into acircular frame.

[0022] In step 300, the sol is gelled and a preform rod is inserted intothe center of the resulting gel. The preform rod is formed by a silicawhich is doped with dopant additives, such as erbium or germanium, tocontrol the characteristics of the optical fiber.

[0023] In step 400, a plurality of glass tubes made of pure silica whosediameter is in the order of 1-2 mm are vertically arranged around thepreform rod in the gel. The glass tubes defines the boundary layers in acladding part to form air holes in the optical fiber.

[0024] In step 500, the gel is removed from the circular frame and driedat constant-temperature in a constant-humidity chamber where apredetermined temperature and predetermined relative humidity aremaintained. After the gel drying step 500, it is preferable to performthe heat treatment at low temperature in accordance with the embodimentof the present invention. The dry gel is loaded into a low-temperatureheat-treatment device and then thermally treated while supplyingchlorine, helium, and oxygen gases in order to dissolve any residualmoisture and/or organic materials (i.e., the binder) and remove anymetallic impurities and hydroxy groups.

[0025] In step 600, the dried gel is glassified through a heatapplication. That is, the gel after the gel drying step 500 (or the lowtemperature heat treatment) is glassified through sintering process athigh temperature, thereby forming a holey optical fiber preform. Thesintering step 600 is performed in a furnace that moves up and downalong the holey opitcal fiber preform obtained from step 500 while beingexposed to an atmosphere of helium gas and at a temperature above 1300°C.

[0026] In step 700, an optical fiber is drawn from the holey opticalfiber preform while supplying gases into one end of the air holes of thepreform and heating the other end of the air holes. The outer and innerair holes of the holey optical fiber preform 10 are scaled down in sizeat a predetermined rate without the shape distortion by supplying apredetermined amount of gas at a constant pressure into the air holes,resulting in uniform air holes in the holey optical fiber. To this end,the supply and optical fiber drawing steps are performed using a holeyoptical fiber fabricating apparatus according to the embodiment of thepresent invention.

[0027]FIG. 3 is a schematic view of the holey optical fiber fabricatingapparatus according to the preferred embodiment of the presentinvention. Referring to FIG. 3, the holey optical fiber fabricatingapparatus is comprised of a preform cover 20, a gas supplier 30, apressure regulator 40, a fixing rod 50, and a stationary heater 60.

[0028] The preform cover 20 seals one end of the preform 10 to prevent aleakage of gas supplied to the preform 10. The fixing rod 50 is attachedto the upper end of the preform cover 20 and is fixed by a chuck (notshown) of an optical fiber drawing device, to fix the holey opticalfiber preform 10 during the drawing of an optical fiber.

[0029] The gas supplier 30 supplies gas into the air holes 16 of theholey optical fiber preform 10 through the preform cover 20. The gas isnitrogen. A gas pipe 52 is coupled to the constant pressure pipe 42 sothat the gas supplied from the gas supplier 30 can be entered into thepreform cover 20.

[0030] The pressure regulator 40 serves to control the gas supplier 30to supply a predetermined pressure inside of the sealed cover 20. Thepressure regulator 40 communicates with the gas pipe 52 via a constantpressure pipe 42. The gas supplied from the gas supplier 30 using themeans of the pressure regulator 40 eliminates errors in size betweenouter air holes and inner air holes, by preventing distortion of the airholes through continuously applying constant pressure to thepre-installed glass tubes defining the air holes 16.

[0031] The heater 60 is disposed at the other end of the holey opticalfiber preform 10 and heats the preform 10 for drawing the optical fiber.A winder (not shown) and a spool (not shown) are installed under theheater 60 to wind a drawn optical fiber.

[0032] In accordance with the apparatus and method for fabricating aholey optical fiber as described above, a predetermined amount of gas isconstantly supplied into the air holes of a holey optical fiber preformduring the drawing of an optical fiber from the preform, therebypreventing distortion of air holes and allowing a mass production ofholey optical fibers with uniform characteristics.

[0033] While the invention has been shown and described with referenceto a certain preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and the scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A method of fabricating a holey optical fiber,comprising the steps of: (a) forming a sol by mixing a startingmaterial, deionized water, and an additive; (b) pouring the sol into acircular frame to form a gel; (c) inserting a preform rod at the centerof the gel; (d) vertically arranging a plurality of glass tubes aroundthe preform rod located in the center of the gel; (e) removing the gelfrom the circular frame to dry the gel; (f) sintering the dried gelunder a heat application to form a preform; and, (g) drawing the holeyoptical fiber from the sintered preform while supplying gas into one endof the sintered preform and heating at the other end.
 2. The method ofclaim 1 , further comprising the step of thermally treating the driedgel at a predetermined temperature to remove impurities from the gelafter executing the step (e).
 3. The method of claim 1 , wherein thepreform rod is formed by an erbium-doped silica.
 4. The method of claim1 , wherein the preform rod is formed by a germanium-doped silica.
 5. Anapparatus for fabricating a holey optical fiber, comprising: a preformcover sealing one end of a holey optical fiber preform; a gas supplierfor supplying gas into the preform cover; a pressure regulator forcontrolling the amount of gas supplied from the gas supplier to beconstant; and, a heating means installed at the other end of the holeyoptical fiber preform for heating the other end of the preform to drawan optical fiber.
 6. The apparatus of claim 5 , further comprising afixing rod attached to the top of the preform cover to hold the holeyoptical fiber preform in a stationary position.
 7. The apparatus ofclaim 5 , wherein the gas is nitrogen.
 8. An apparatus for fabricating aholey optical fiber, comprising: a tubular preform; a sealing meansoperative to cover the top portion of the tubular preform for receivinga flow of gas at a predetermined pressure; a storage means to store thegas; a regulating means for controlling the amount of gas supplied fromthe storage means to the sealing means to be constant; and, a heatingmeans coupled at the other end of the tubular preform for heating thetubular preform while drawing an optical fiber from the tubular preform.9. The apparatus of claim 8 , wherein the tubular preform is formed bythe following steps: (a) forming a sol by mixing a starting material,deionized water, and an additive; (b) pouring the sol into a circularframe to form a gel; (c) inserting a preform rod at the center of thegel; (d) vertically arranging a plurality of glass tubes around thepreform rod located in the center of the gel; (e) removing the gel fromthe circular frame to dry the gel; and, (f) sintering the dried gelunder a heat application to obtain the tubular preform.
 10. Theapparatus of claim 8 , wherein the gas is nitrogen.